The present invention relates to a raster scan type CRT (Cathode-Ray Tube) display system.
A raster scan type CRT display system usually includes a refresh memory of a capacity equal to the number of a field of characters displayed, and coded data in the refresh memory is read out character-by-character under the control of a timing control circuit for displaying them on a CRT viewer.
When displayed data is to be read or written by a processor, an address of the refresh memory is designated by a cursor register (See, for example, U.S. Pat. No. 3,771,155 to Yukitaka Hayashi et al entitled "Color Display System"). When more information is to be displayed, a large size CRT and a large capacity refresh memory are required. This leads to considerable increase in cost. Furthermore, because of a practical limit of the size of the CRT, it is difficult to obtain a large size CRT capable of displaying more than several thousands characters at a reasonable cost.
As one approach to accomplish the display of a large amount of information with a limited size CRT and a limited capacity refresh memory, there is the so-called image rolling technique in which displayed characters are moved during display.
According to this method, in the case of rolling up, for example, the displayed characters are shifted upwardly line-by-line while additionally displaying new data in the lowermost line so that a large number of characters exceeding in number the one field of characters displayed can be displayed.
When the rolling of the displayed image is to be effected, it is necessary to relatively shift the character position address on the screen and the read address of the refresh memory. Furthermore, in reading or writing data by the processor for the shifted image, the address must be designated taking the shift of the read address into consideration.
Heretofore, no satisfactory solution for such requirements has been known.